Aluminum die casting alloy, aluminum die cast product and production process

ABSTRACT

An aluminum alloy for die casting, contains, in terms of mass percentage: Si in the range of 1.0˜3.5 %; Mg in the range of 2.5˜4.5 %; Mn in the range of 0.3˜1.5%; Fe in the range equal to or less than 0.15%; Ti in the range of equal to or less than 0.20%; and the balance of aluminum and inevitable impurities. A die cast product having good strength and elongation in a high strain rate region is produced from the aluminum alloy without the need for solution heat treatment.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to aluminum die casting alloy providing superior mechanical properties and superior castability, specifically resistance to cracking during casting, without requiring solution heat treatment after die casting, and to die cast product and production process based on such an aluminum alloy.

[0002] Die casting process is widely used for producing parts of automotive engines and transmissions because of its capability of casing into thin wall shapes, high dimensional accuracy, high productivity and flexibility in design of shape. Recently, nodes of space frame structures, and center pillars of vehicle bodies, for example, are enrolled as parts which can be made from aluminum alloy die castings improved in mechanical properties such as tensile strength, 0.2% offset yield strength, and elongation, by heat treatment after vacuum die casting process.

[0003] Published Japanese Patent Application Publication (Kokai) No. 8-41575 discloses die casting aluminum alloy of Al—Si—Mg—Mn system usable for parts of motor vehicles. Furthermore, Published Japanese Patent Applications Publication (Kokai) Nos. 5-9638, 11-293375, and 11-80875; and document “Use of Low Iron Die Casting Alloys for the Automotive Industry”, Properties and Applications, DIE CASTING ENGINEER January/February 1998) disclose die casting aluminum alloys requiring no solution heat treatment after die casting.

SUMMARY OF THE INVENTION

[0004] The solution heat treatment and aging process performed after die casting to achieve desired mechanical properties are problematical in that the further need for an operation for correcting distortion produced during water cooling after solution heat treatment tends to deteriorate productivity and increase production cost. Moreover, Al—Mg—Mn(—Zr) system alloys recited in the above documents are susceptible to cracking during casting even though the need for post-casting heat treatment is eliminated.

[0005] For vehicle parts such as A pillar, B pillar, C pillar, roof, space frame joint, and suspension mount, important features are stable high strength and high elongation even in a high speed deformation region to ensure the safety in case of collision. In such applications, however, conventional technology deals merely with static strength and elongation, and there remain problems in a high strain rate region at a level of 1000/s.

[0006] It is an object of the present invention to provide compositions of material, manufactures and production processes about aluminum die casting alloys which are advantageous in production, and superior in mechanical properties especially in high strain rate region.

[0007] It is another object of the present invention to provide compositions, products and production processes of aluminum die casting alloys which can be used as final product without requiring solution heat treatment after die casting process, which can provide high strength and elongation stably especially in high strain rate region as in collision of vehicles, to a level adequate as material for vehicle body parts such as A, B and C pillars, roofs, joints for space frames, and mounting parts for suspension, and which is superior in castability, especially in resistance to cracking during casting.

[0008] According to the present invention, an aluminum alloy for die casting, comprises, in terms of mass ratio: Si in the range of 1.0˜3.5%; Mg in the range of 2.5˜4.5%; Mn in the range of 0.3˜1.5%; Fe in the range equal to or less than 0.15%; a further constituent comprising Ti in the range of equal to or less than 0.20%; and the balance of Al and inevitable impurities. The further constituent may consist of titanium, and the aluminum alloy may consist essentially, in terms of mass ratio, of: silicon in the range of 1.0˜3.5%; magnesium in the range of 2.5˜4.5%; manganese in the range of 0.3˜1.5%; iron in the range equal to or less than 0.15%; titanium in the range of equal to or less than 0.20%; and the balance of Al and inevitable impurities. The further constituent may consist of Ti and B, and the aluminum alloy may consist essentially, in terms of mass ratio, of: Si in the range of 1.0˜3.5%; Mg in the range of 2.5˜4.5%; Mn in the range of 0.3˜1.5%; Fe in the range equal to or less than 0.15%; Ti in the range of 0.05˜0.20%; B in the range of 0.001˜0.10%; and the balance of Al and inevitable impurities. The further constituent may consist of Ti and Zr, and the aluminum alloy may consist essentially, in terms of mass ratio, of Si in the range of 1.0˜3.5%; Mg in the range of 2.5˜4.5%; Mn in the range of 0.3˜1.5%; Fe in the range equal to or less than 0.15%; Ti in the range equal to or smaller than 0.20%; Zr in the range of 0.05˜0.30%; and the balance of Al and inevitable impurities.

[0009] According to the present invention, a production process comprises: preparing an aluminum alloy comprising, in terms of mass ratio, Si in the range of 1.0˜3.5%, Mg in the range of 2.5˜4.5%, Mn in the range of 0.3˜1.5%, Fe in the range equal to or less than 0.15%, a further constituent comprising Ti in the range equal to or less than 0.20%, and the balance of Al and inevitable impurities; and forming the aluminum alloy into a fixed shape by a die casting operation.

[0010] The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a perspective view showing a cast product according to the present invention, used for evaluation of cracking in a first test.

[0012]FIG. 2 is a perspective view illustrating the positions of cracks formed in a casting as shown in FIG. 1.

[0013]FIG. 3 is a graph for illustrating the influence of Si content on resistance to cast cracking.

[0014]FIG. 4 is a plan view showing the shape of die cast material formed by casting in a second test according to the present invention.

[0015]FIG. 5 is a plan view showing the shape of a test piece used in static tensile test of the die cast material shown in FIG. 4.

[0016]FIG. 6 is a plan view showing the shape of a test piece used in dynamic tensile test of the die cast material shown in FIG. 4.

[0017]FIG. 7 is a schematic view showing a one-bar method high speed tensile test machine used in the dynamic tensile test.

[0018]FIG. 8 is a graph showing a typical stress-strain curve in the dynamic tensile test.

[0019]FIG. 9 is a graph showing the influence on the strength of each alloy, of the Mg/Si ratio in Test 2.

[0020]FIG. 10 is a graph showing the influence on the elongation of each alloy, of the Mg/Si ratio in Test 2.

[0021]FIG. 11 is a graph showing the influence on the strength of each alloy, of the Mg/Si ratio in Test 3.

[0022]FIG. 12 is a graph showing the influence on the strength of each alloy, of the Mg/Si ratio in Test 3.

[0023]FIG. 13 is a graph showing variation in hardness of the alloys of practical example 13 and practical example 16 according to the present invention while being held constantly at 250° C.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The following is explanation on conditions for alloy compositions and heat treatment according to the present invention.

[0025] (1) Si: 1.0˜3.5% Silicon is an element effective in improving resistance to cracking at the time of die casting. However, the effect is insufficient when the Si content is lower than 1.0%. When, on the other hand, the silicon content is beyond 3.5%, the elongation becomes lower in the high strain rate region because of crystallization of intermetallic compounds of Mg—Si system in great quantities. Therefore, the Si content is in the range equal to or more than 1.0%, and equal to or less than 3.5%. The Si content range of 2.5˜3.5% is preferable when the emphasis is on the improvement in resistance to cracking. When the emphasis is to be laid on the improvement in elongation in the high strain rate region, the Si content range of 1.0˜2.0% is preferable.

[0026] (2) Mg: 2.5˜4.5% Magnesium is an element effective for improving the strength by being dissolved in a matrix of aluminum alloy and by forming Mg₂Si with coexisting Si. The improvement in the strength is insufficient when the Mg content is less than 2.5%. When the Mg content is more than 4.5%, the tendency to stress corrosion cracking increases. Therefore, the Mg content in the range equal to or more than 2.5%, and equal to or less than 4.5% is effective in improving the strength and at the same time restraining the stress corrosion cracking.

[0027] (3) Mn: 0.3˜1.5% Manganese is an element effective for improving the high temperature strength by forming crystals of Al₆Mn during solidification, restraining deformation of a die cast product at the time of removal of the product after die casting, and for restraining the occurrence of soldering (or sticking) to the die casting dies. The effects for the high temperature strength and prevention of soldering to dies are insufficient when the Mn content is less than 0.3%. When the Mn content is more than 1.5%, the elongation especially in the high strain rate region becomes smaller because of crystallization of coarse-grained intermetallic compound of the AL-Mn system. Therefore, the Mn content in the range equal to or more than 1.0%, and equal to or less than 1.5% is effective. The Mn content range of 1.0˜1.5% is preferable for ensuring these effects of the addition of Mn.

[0028] (4) Fe: 0.15% or less Iron is an element effective in preventing soldering (or sticking) to metal die during die casting. However, an increase in the Fe content increases the amount of crystallization of needle intermetallic compound of Fe system, and thereby decreases the elongation and toughness. Therefore, the Fe content range is equal to or less than 0.15%.

[0029] (5) Ti: 0.20% or less; or Ti: 0.05˜0.20% and B: 0.001˜0.10% Titanium, or titanium and boron is additive effective in reducing a grain size of primary α (Al) crystal phase and thereby improving mechanical properties of die castings. The effect is insufficient when the Ti content is less than 0.1% in the case of Ti alone, and when the Ti content is less than 0.05% and the B content is less than 0.001% in the case of combined addition of Ti and B. When the Ti content is more than 0.20% or the B content is more than 0.10%, TiAl₃ particles or TiB₂ particles become coarse, and the grain size refinement effect is reduced. Therefore, to ensure these effects, it is desirable to set the Ti content in the range equal to or more than 0.10%, and less than or equal to 0.20%, or to set the Ti content in the range more than or equal to 0.05%, and less than or equal to 0.20%, and the B content in the range more than or equal to 0.001% and less than or equal to 0.10%.

[0030] (6) Zr: 0.05˜0.30% Zirconium is also effective in grain size refinement of the primary α (Al) crystal phase and thereby improving mechanical properties of die castings. Accordingly, it is possible to add Zr according to the need. The effect is insufficient when the Zr content is less than 0.05%. When the Zr content is more than 0.30%, the tendency to coarse Al—Zr intermetallic compound increases and the elongation (especially in the high strain rate region) becomes lower. Therefore, if Zr is added, the Zr content is set in the range of 0.05%˜0.30%.

[0031] (7) Mg/Si ratio: 2.0 or less The mechanical properties and age hardening are influenced by the Mg/Si ratio which is the ratio of the Mg content to the Si content (Mg content/Si content). When the Mg/Si ratio is smaller than the ratio corresponding to the composition of Mg₂Si (that is, the Si content is great as compared to the Mg content), Si particles tend to crystallize at the time of solidification, and the strength is increased. Therefore, the ratio of the Mg content to the Si content is set equal to or less than 2.0 to achieve a high strength in the as-cast state.

[0032] (8) Mg/Si ratio: 1.8 or more When the Mg/Si ratio is greater than the ratio corresponding to the composition of Mg₂Si (that is, the Si content is small as compared to the Mg content), an excess amount of Mg acts to decrease the solubility of Mg₂Si. Therefore, the strength decreases whereas the ductility improves. In order to achieve high ductility in the as-cast state, the Mg/Si ratio is set equal to less than 1.8.

[0033] Artificial aging or stabilizing operation: 130° C.˜300° C. When the Mg/Si ratio is greater than the balanced ratio of the composition (that is, the Si content is relatively low as compared to the Mg content), the solid solubility of Mg₂Si is low and hence the amount of excess Mg₂Si over saturation is high, so that the alloy exhibits the age hardening effect by heat input after die casting. Accordingly, when higher strength is demanded, it is possible to obtain a desired characteristic by heat treatment in the temperature range equal to higher than 130° C., and equal to or lower than 300° C. for a predetermined time duration. This age hardening effect is achieved by the application of heat to a die cast product. Therefore, it is possible to attain the same effect by annealing operation to remove internal stresses after die casting or by baking in coating process, instead of the artificial age hardening operation or the stabilizing operation.

[0034] Stabilizing operation: 150° C.˜400° C. When the Mg/Si ratio is smaller than the balanced ratio (that is, the Si content is relatively high with respect to the Mg content), the solid solubility of Mg₂Si is hardly decreased, and hence the age hardening effect hardly appears. By the stabilizing operation, however, it is possible to control the configuration of crystals and thereby to improve the ductility. When a high ductility is desired, it is possible to achieve a desired characteristic by performing the stabilizing operation in the temperature range equal to or higher than 150° C., and equal to or lower than 400° C.

[0035] The die casting aluminum alloy according to the present invention having the above-mentioned composition can provide sufficient strength due to solid solution strengthening by the addition of Mg and Mn and precipitation by the addition of Mg and Si. Besides, sufficient ductility is achieved by the limitation of Fe detrimental to elongation within predetermined limits is effective in ensuring elongation, and the addition of Mn to minimize the undesired influence of Fe. Therefore, this die casting aluminum alloy enables stable production of aluminum die cast products having high strength and elongation even in high strain rate condition, without the need for solution heat treatment after casting. Accordingly, this die casting aluminum alloy is adequate for automobile body parts such as A pillar, B pillar, C pillar, roof, space frame joint, suspension fitting part, suspension arm, sub-frame, and suspension link.

[0036] In producing aluminum die cast products by using this die casting aluminum alloy, a high vacuum die casting process is a preferable process. The high vacuum die casting process is advantageous in reducing involvement of gases in casting, so that it is possible to make full use of superior mechanical properties, specifically elongation and toughness.

[0037] Moreover, it is optional to employ artificial aging process or stabilizing treatment after the die casting. It is possible to replace the artificial aging or stabilizing treatment by either or both of annealing operation and coating layer baking operation.

[0038] The aluminum die casting alloy according to the present invention can provide superior static and dynamic strength and elongation without the need for heat treatment. However, in a case where further improvement in strength or balance between strength and elongation is desired, the application of artificial aging process is effective in adjusting desired mechanical properties.

[0039] An aluminum alloy according to the present invention comprises, as alloying elements, 1.0˜3.5% Si; 2.5˜4.5% Mg; 0.3˜1.5% Mn; 0.15% or less Fe; and 0.20% or less Ti. Therefore, the aluminum alloy can provide sufficient strength by the solid solution strengthening by the addition of Mg, precipitation hardening and natural ageing. At the same time the aluminum alloy can ensure superior elongation by reduction of Fe detrimental to elongation, to the minimum level. Moreover, the addition of Ti further improves the strength and elongation by the grain size refinement of primary α (Al) crystals.

[0040] Titanium is an ingredient for further improving mechanical properties of die castings, such as strength and elongation, by grain-refinement of the primary α (Al) phase of the alloy. This ingredient may contain Ti only in the range of 0.10˜0.20% (mass ratio); or may contain Ti in the range of 0.05˜0.20% (mass ratio) and B in the range of 0.001˜0.10% (mass ratio); or may contain Ti in the range of Ti in the range of 0.20% or less (mass ratio) and Zr in the range of 0.05˜0.30% (mass ratio). Therefore, this ingredient further improves mechanical properties securely by the grain size refinement.

[0041] By the limitation of the Mn content to the range of 1.0˜1.5% (mass ratio), it is possible to ensure the function of Mn in the alloy, and thereby to further improve the strength at high temperatures and elongation especially in the high strain rate region. The Si content may be limited to the range of 2.5˜3.5 (mass ratio). This range is effective in ensuring the function of Si, and improving the resistance to cast cracking. The further limited Si content range of 1.0˜2.0% (mass ratio) is effective in improving the elongation in the high strain rate region.

[0042] The Mg/Si ratio range equal to or less than 2.0 is effective in promoting crystallization of Si particles at the time of solidification and thereby to provide sufficient as-cast strength.

[0043] The Mg/Si ratio range equal to or more than 1.8 is effective in obtaining high as-cast ductility though the strength is decreased slightly.

[0044] Practical Examples Practical examples according to the present invention are as follows:

[0045] TEST 1 Molten aluminum alloy samples of practical examples were prepared at 750°. The compositions are listed in TABLE 1. Thereafter, bubbling operation was performed with argon gas for removal of inclusions and degassing. In Table 1, samples are alloy samples of practical examples 1˜5, and comparatives are alloy examples of comparative examples 1˜4. TABLE 1 CHEMICAL COMPOSITION (MASS %) ALLOY Si Mg Mn Fe Ti Al Mg/Si SAMPLE 1 1.1 4.0 0.9 0.13 0.04 BAL. 3.63 2 1.5 4.1 1.1 0.12 0.05 BAL. 2.73 3 2.0 4.0 1.1 0.13 0.04 BAL. 2.00 4 2.5 4.0 1.0 0.12 0.05 BAL. 1.60 5 3.5 3.9 1.1 0.14 0.05 BAL. 1.11 COMPAR- 1 0.4 4.2 1.1 0.13 0.06 BAL. 10.5 ATIVE 2 0.8 4.1 1.1 0.12 0.05 BAL. 5.13 3 5.0 4.1 1.2 0.11 0.06 BAL. 0.82 4 9.5 4.2 1.0 0.15 0.04 BAL. 0.44

[0046] Then, the molten aluminum alloy samples were self-cooled to 720° C., respectively, and thereafter cast into a form as shown in FIG. 1 by gravity casting with an iron mold for evaluating the characteristic of cast cracking.

[0047] After the gravity casting, each sample was cooled in the mold until the temperature of the obtained casting was decreased to about 100° C. or lower. Then, the cast cracking characteristic was evaluated by counting the numbers of cracks formed as schematically shown in FIG. 2. FIG. 3 shows the results arranged in the form of the number of cracks with respect to the Si content. The results of FIG. 3 reveal that by increasing the Si content, it is possible to restrain the number of cracks, and to improve the tendency to cracking. When the Si content is increased to 1.1%, the number of cracks is decreased to ½ of the number of cracks at an Si content of 0.4%. When the Si content is 2.5% or more, the number of cracks is decreased to zero. In FIG. 3, samples 1˜5 stand for practical examples 1˜5 according to the present invention, respectively, and comparatives 1˜4 stand for comparative examples 1˜4.

[0048] TEST 2 Molten aluminum alloy samples were prepared at 750°. The compositions are listed in TABLE 2. Samples 1, 2 and 4˜10 stand for practical examples 1, 2 and 4˜10 according to the present invention, and comparatives stand for comparative examples 1, 2, 3, 5 and 6. Thereafter, bubbling operation was performed with argon gas for removal of inclusions and degassing. TABLE 2 CHEMICAL COMPOSITION (MASS %) ALLOY Si Mg Mn Fe Ti B Zr Al Mg/Si SAMPLE 1 1.1 4.0 0.9 0.13 0.04 — — BAL. 3.63 2 1.5 4.1 1.1 0.12 0.05 — — BAL. 2.73 4 2.5 4.0 1.0 0.12 0.05 — — BAL. 1.60 5 3.5 3.9 1.1 0.14 0.05 — — BAL. 1.11 6 2.5 4.0 1.1 0.11 0.15 — — BAL. 1.60 7 2.7 4.2 1.0 0.10 0.11 0.03 — BAL. 1.56 8 2.7 4.0 1.1 0.01 0.03 — 0.20 BAL. 1.48 9 1.5 4.0 0.6 0.12 0.05 — — BAL. 2.67 10 1.4 3.9 0.3 0.13 0.06 — — BAL. 2.79 COMPAR- 1 0.4 4.2 1.1 0.13 0.06 — — BAL. 10.5 ATIVE 2 0.8 4.1 1.1 0.12 0.05 — — BAL. 5.13 3 5.0 4.1 1.2 0.11 0.06 — — BAL. 0.82 5 2.0 5.2 0.6 0.12 0.15 — — BAL. 2.60 6 1.5 3.9 0.1 0.13 0.04 — — BAL. 2.60

[0049] Then, each aluminum alloy was die-cast by using a high vacuum die casting machine having a clamping force of 320 tons after the application of powder parting agent to the die under the conditions of a casting pressure of 60 MPa, a high injecting speed of 3.5 m, a degree of vacuum in sleeve of 0.96 atmosphere, and a degree of vacuum at vacuum valve of 0.95 atmosphere. The temperature of molten alloy at the time of casting was 680° C. This test employed a die having a cavity shaped like a flat plate of 50 mm×130 mm×2 mm as shown in FIG. 4.

[0050] From the thus-produced plate-shaped die casting, a test piece of JIS 13B as shown in FIG. 5 was cut out and subjected to static tensile test. The static tensile test was carried out with Instron universal testing machine at a strain rate of 0.001/s.

[0051] From the above-mentioned plate-shaped die casting, a test piece shown in FIG. 6 was cut out and subjected to dynamic tensile test. The dynamic tensile test was carried out with one-bar method high speed tensile testing machine as shown in FIG. 7 at a strain rate of 1000/s. TABLE 3 shows the results of the static tensile test and the dynamic tensile test. In both of the static and dynamic tensile tests, the number of repetitions was five, and each result was an average of five values obtained by the five repetitions. In the dynamic tensile test, the stress varies as shown in a stress-strain diagram of FIG. 8. Accordingly, the strength of each die casting was evaluated in terms of a value of the post-yielding peat stress (max stress after yield) as indicated in FIG. 8. TABLE 3 DYNAMIC TENSILE TEST STATIC TENSILE TEST MAX 0.2% STRESS TENSILE YIELD AFTER STRENGTH STRENGTH ELONGATION YIELD ELONGATION ALLOY (MPa) (MPA) (%) (MPa) (%) SAMPLE 1 245 124 24.6 277 26.8 2 244 126 24.3 279 25.9 4 249 134 23.8 284 23.9 5 255 144 21.2 302 20.9 6 251 140 24.1 291 24.0 7 255 143 24.2 305 23.9 8 250 139 23.8 298 23.4 9 242 125 24.7 283 26.1 10 242 122 25.1 280 26.4 COMPAR- 1 246 121 25.2 272 27.3 ATIVE 2 246 122 24.6 275 27.0 3 253 145 16.8 271 15.5 5 261 158 20.5 312 19.5 6 240 122 25.5 273 26.7

[0052] As shown by the results in TABLE 3, the aluminum die casting material according to the practical examples of the present invention can provide superior static and dynamic mechanical properties. Presumably, this effect is attributable to the following factors. The strength of solid solution is increased by addition of Mg and Mn. The addition of Mg and Si improves the strength by precipitation. The elongation is improved by minimizing adverse influence of Fe by reduction of Fe content and addition of Mn. Moreover, it is confirmed that the additive of Ti, Ti+B or Zr improves the mechanical properties in both of the static and dynamic conditions. The results of TABLE 3 show that in the range of Si addition quantity beyond 3.5%, there is a tendency for elongation to decrease in the dynamic tensile test. No or little influence is exerted on the strength by increase or decrease of the Mn additive quantity, but the elongation tends to improve as the Mn additive quantity decreases. However, in the samples in which the Mn content is lower than 0.3%, there is a tendency to soldering, or sticking or burning to die during die casting.

[0053]FIGS. 9 and 10 show the results of the tensile test shown in FIG. 3 in the form of relationship with respect to an Mg/Si ratio between the Mg content and the Si content. As shown in graphs of FIGS. 9 and 10, there is a tendency to greater elongation and smaller strength in a range of the Mg/Si ratio greater than a boundary defined by an Mg/Si ratio of about 2.0. In a range smaller than the boundary of the Mg/Si ratio of about 2.0, there is a tendency to smaller elongation and greater strength. In this example, the Si content was varied largely so that it is not adequate to conclude that the Mg/Si ratio is causative. Therefore, further test was carried out in detail.

[0054] TEST 3 Molten aluminum alloy samples were prepared at 750° as in the preceding practical examples. TABLE 4 shows the compositions of samples in practical examples 11˜16 according to the present invention. Thereafter, bubbling operation was performed with argon gas for removal of inclusions and degassing. TABLE 4 CHEMICAL COMPOSITION (MASS %) Mg/ ALLOY Si Mg Mn Fe Ti B Zr Al Si SAMPLE 11 1.6 2.88 1.25 0.04 0.05 — — BAL. 1.80 12 1.5 2.8 1.27 0.13 0.04 — — BAL. 1.87 13 2.0 3.5 1.29 0.04 0.05 — — BAL. 1.75 14 1.6 3.3 1.21 0.18 0.05 — — BAL. 2.06 15 1.6 3.5 1.30 0.18 0.04 — — BAL. 2.19 16 1.5 4.4 1.28 0.14 0.05 — — BAL. 2.27

[0055] Then, each aluminum alloy was die-cast by using a high vacuum die casting machine having a clamping force of 320 tons after the application of powder parting (or releasing) agent to the die, under the conditions of a casting pressure of 60 MPa, a high injecting speed of 3.5 m, a degree of vacuum in sleeve of 0.96 atmosphere, and a degree of vacuum at vacuum valve of 0.95 atmosphere. The temperature of molten alloy at the time of casting was 860° C. This test employed a die having a cavity shaped like a flat plate of 50 mm×130 mm×2 mm as shown in FIG. 4. From the thus-produced plate-shaped die casting, a test piece of JIS 13B as shown in FIG. 5 was cut out and subjected to static tensile test. The static tensile test was carried out with Instron universal testing machine at a strain rate of 0.001/s.

[0056] As shown in FIG. 11, the strength (0.2% offset yield strength) of the aluminum alloy according to the present invention exhibits a tendency to increase in the range of Mg/Si ratio lower than or equal to 2.0 without regard to the amount of the Mg content. In this range, the Si content is relatively high with respect to the Mg content, so that the strength is increased by crystallization of Si grains during solidification. On the other hand, as shown in FIG. 12, the elongation of the aluminum alloy according to the present invention exhibits a tendency to decrease sharply in the range of Mg/Si ratio lower than 1.8 without regard to the amount of Mg content. In this range, the Si content is relatively low with respect to the Mg content, so that the solid-solubility of Mg₂Si decreases because of the existence of excess Mg, and the strength decreases whereas the elongation improves. Therefore, in order to achieve high strength in the as-cast state, it is desirable to adjust the Mg content and the Si content so as to make the Mg/Si ratio lower than or equal to 2.0. In order to achieve high ductility in the as-cast state, it is desirable to adjust the Mg content and the Si content so as to make the Mg/Si ratio greater than or equal to 1.8.

[0057]FIG. 13 shows the results of measurement of the hardness of the alloy of the practical example 13 having a relatively small Mg/Si ratio and the alloy of the practical example 16 having a relatively great Mg/Si ratio while held at constant temperature of 250° C. As shown in FIG. 13, the hardness decreases monotonically with time by the thermal load in the case of the material of the practical example 13 having the smaller Mg/Si ratio. However, the material of the practical example 16 having the greater Mg/Si ratio exhibits such behavior of age hardening that the hardness increases first by thermal load and then decreases. In the case of the alloy having the greater Mg/Si ratio, the solid solubility of Mg₂Si is decreased by the existence of excess Mg, and Mg₂Si tends to precipitate from a solid solution supersaturated with Mg₂Si, by heat input. Thus, in the alloy according to the present invention, it is possible to control the balance between strength and elongation by controlling the Mg/Si ratio and the input of heat after die casting.

[0058] This application is based on Japanese Patent Application No. 2000-325756 filed in Japan on Oct. 25, 2000, and Japanese Patent Application No.2001-238947 filed in Japan on Aug. 7, 2001. The entire contents of these Japanese Patent Applications are hereby incorporated by reference.

[0059] Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in light of the above teachings. The scope of the invention is defined with reference to the following claims. 

What is claimed is:
 1. An aluminum alloy for die casting, comprising, in terms of mass ratio: silicon in the range of 1.0˜3.5%; magnesium in the range of 2.5˜4.5%; manganese in the range of 0.3˜1.5%; iron in the range equal to or less than 0.15%; a further constituent comprising titanium in the range of equal to or less than 0.20%; and the balance of aluminum and inevitable impurities.
 2. The aluminum alloy as claimed in claim 1, wherein the further constituent consists of titanium, and the aluminum alloy consists essentially, in terms of mass ratio, of: silicon in the range of 1.0˜3.5%; magnesium in the range of 2.5˜4.5%; manganese in the range of 0.3˜1.5%; iron in the range equal to or less than 0.15%; titanium in the range of equal to or less than 0.20%; and the balance of aluminum and inevitable impurities.
 3. The aluminum alloy as claimed in claim 2, wherein the Ti content is 0.10% or more.
 4. The aluminum alloy as claimed in claim 1, wherein the further constituent consists of titanium and boron, and the aluminum alloy consists essentially, in terms of mass ratio, of: silicon in the range of 1.0˜3.5%; magnesium in the range of 2.5˜4.5%; manganese in the range of 0.3˜1.5%; iron in the range equal to or less than 0.15%; titanium in the range of 0.05˜0.20%; boron in the range of 0.001˜0.10%; and the balance of aluminum and inevitable impurities.
 5. The aluminum alloy as claimed in claim 1, wherein the further constituent consists of titanium and zirconium, and the aluminum alloy consists essentially, in terms of mass ratio, of silicon in the range of 1.0˜3.5%; magnesium in the range of 2.5˜4.5%; manganese in the range of 0.3˜1.5%; iron in the range equal to or less than 0.15%; titanium in the range equal to or smaller than 0.20%; zirconium in the range of 0.05˜0.30%; and the balance of aluminum and inevitable impurities.
 6. The aluminum alloy as claimed in claim 1, wherein the Mn content is equal to or more than 1.0%.
 7. The aluminum alloy as claimed in claim 1, wherein the Si content is equal to or more than 2.5%.
 8. The aluminum alloy as claimed in claim 1, wherein the Si content is equal to or less than 2.0%.
 9. The aluminum alloy as claimed in claim 1, wherein a ratio of the Mg content in mass %, to the Si content in mass % is equal to or smaller than 2.0.
 10. The aluminum alloy as claimed in claim 1, wherein a ratio of the Mg content in mass %, to the Si content in mass % is equal to or greater than 1.8.
 11. A product comprising an aluminum die casting of the aluminum alloy as claimed in claim
 1. 12. The product as claimed in claim 11, wherein the product is a part of a motor vehicle.
 13. A production process comprising: preparing an aluminum alloy comprising, in terms of mass ratio, silicon in the range of 1.0˜3.5%, magnesium in the range of 2.5˜4.5%, manganese in the range of 0.3˜1.5%, iron in the range equal to or less than 0.15%, a further constituent comprising titanium in the range equal to or less than 0.20%, and the balance of aluminum and inevitable impurities; and forming the aluminum alloy into a fixed shape by die casting operation.
 14. The production process as claimed in claim 13, wherein the die casting is vacuum die casting.
 15. The production process as claimed in claim 13, wherein a ratio of the Mg content in mass % to the Si content in mass % of the aluminum alloy is equal to or greater than 1.8; and the production process further comprises heat-treating a die-casting obtained by the die casting operation, in the temperature range of 130° C.˜300° C. after the die casting operation.
 16. The production process as claimed in claim 15, wherein the operation of heat-treating is one of an artificial aging operation and a stabilizing operation.
 17. The production process as claimed in claim 15, wherein the operation of heat-treating comprises at least one of an annealing operation, and a baking operation of baking a coating layer.
 18. The production process as claimed in claim 13, wherein a ratio of the Mg content in mass % to the Si content in mass % of the aluminum alloy is equal to or smaller than 2.0; and the production process further comprises stabilizing treatment in the temperature range of 150° C.˜400° C. of stabilizing a die-casting obtained by the die casting operation.
 19. A product comprising an aluminum die casting produced by the production process as claimed in claim
 13. 20. The product as claimed in claim 19, wherein the product is one of A pillar, B pillar, C pillar, roof, space frame node, and fitting for suspension of a motor vehicle.
 21. The product as claimed in claim 19, wherein the product is one of suspension arm, sub-frame and link for suspension. 